DotNetCoreTest fails to use XUnit2Settings on Cake 0.27

### What You Are Seeing?

I'm using the `DotNetCoreTest` command using both `DotNetCoreTestSettings` as well as `Xunit2Settings`:

```
DotNetCoreTest(dotnetTestSettings, project, xunitSettings);
```

which results in the following exception:

```
Error: One or more errors occurred.
Method not found: 'Cake.Common.Tools.XUnit.ParallelismOption Cake.Common.Tools.XUnit.XUnit2Settings.get_Parallelism()'.
```

### What is Expected?

Expected is that the tests will be executed.

### What version of Cake are you using?

The error occurs in 0.27.0 and 0.27.1.
The error does _not_ occur in 0.26.1.

I think the regression has been introduced by the change of `ParallelismOption` to a nullable.

See https://github.com/cake-build/cake/pull/2040/files#diff-133ec58ad83fd466eed3b129d32daefaL84

### Are you running on a 32 or 64 bit system?

64 bit

### What environment are you running on? Windows? Linux? Mac?

Windows

### Are you running on a CI Server? If so, which one?

Yes, Jenkins.

### How Did You Get This To Happen? (Steps to Reproduce)

See "What You Are Seeing?".

###